The reciprocating piston spark ignition engine is one known form of internal combustion engine used to power automotive vehicles. Reciprocating piston spark ignition engines comprise a number of pistons arranged to reciprocate in respective cylinders and each connected to a crankshaft. Each of the cylinders is provided with inlet valving for controlling the inflow of air and fuel, exhaust valving for controlling the exhaust of the products of combustion and a spark plug for igniting the air fuel mixture. Where the supply of fuel to the engine is controlled by a carburettor, the air and fuel are mixed in an intake manifold upstream of the cylinders and the inlet valving comprises an intake valve that controls the intake of the fuel-air mixture into the cylinder. If the fuel supply to the cylinders is by fuel injection, the inlet valving comprises two valves. One of the valves is a fuel injector and the other is an air intake valve. The fuel injector may be arranged to inject fuel directly into the cylinder or may inject it into an air intake duct just upstream of the air intake valve.
Typically, reciprocating spark ignition engines operate a four-stroke cycle. Each movement of a piston up or down its cylinder comprises one stroke of the four-stroke cycle. The four-stroke cycle consists of:    an induction stroke during which the inlet valving opens and air and fuel are taken into the engine as the piston moves towards the crankshaft;    a compression stroke during which the inlet and exhaust valving are closed and the air fuel mixture is compressed while the piston moves away from the crankshaft;    a power, or working, stroke during which the compressed mixture is ignited and the rapid expansion caused by combustion of the mixture forces the piston back towards the crankshaft; and    an exhaust stroke during which the exhaust valving is open and the exhaust gases are forced out of the cylinder as the piston moves away from the crankshaft again.
Some reciprocating piston spark ignition engines operate a two-stroke cycle, which is a variant of the four-stroke cycle. Such engines are usually of smaller capacity than four-stroke engines and in terms of passenger vehicles tend to be used for two-wheeled vehicles. Two stroke engines use ports located along the side of the cylinder instead of valves. As the piston moves up and down the cylinder, the ports are covered and uncovered depending on where the piston is in the cylinder. In essence, in a two-stroke engine the induction and compression processes take place during the first stroke and the combustion and exhaust processes take place during the second stroke.
The reciprocating piston compression ignition internal combustion engine is another form of engine commonly used to power automotive vehicles. Reciprocating piston compression ignition engines use a fuel having a higher auto-ignition temperature than the fuels used by spark ignition engines and operate a modified version of the four-stroke cycle described above. Specifically, during the induction stroke air is drawn into the cylinder and that air is compressed to a high pressure and temperature during the compression stroke. Fuel is then injected directly into the cylinder (or into a mixing chamber that leads into the cylinder) and combustion takes place as the fuel mixes with the high temperature compressed air in the cylinder. Historically, reciprocating piston compression ignition engines were considered noisy and slow and in the automotive field were used mainly for trucks and other commercial vehicles such as buses. However in more recent times, high performance reciprocating piston compression ignition engines have been developed and now reciprocating piston compression ignition engines are commonly used in small passenger vehicles such as saloon cars (sedans).
The Wankel engine is another form of spark ignition engine that has been used to power automotive vehicles. The Wankel engine employs a four ‘stroke’ cycle similar to the four-stroke cycle employed by the reciprocating piston spark ignition internal combustion engine. However, instead of reciprocating pistons, the Wankel engine has a roughly triangular rotor that is mounted on an eccentric shaft for rotation in an approximately oval (epitrochoid-shaped) chamber. The ‘four strokes’ take place in the spaces between the rotor and the chamber wall.
A common feature of these known internal combustion engines is that the fuel air mixture is input to a chamber in which it is combusted so that the rapid expansion of the mixture caused by the combustion acts directly on a body (piston or rotor) that is connected to an output shaft so as to cause rotation of the shaft; the output of the engine being the rotation of the shaft.
An engine in which a body of liquid acts as the reciprocating piston has also been proposed.